Explanation: When passing Earth on your way to Jupiter, what should you look for? That question arose for the robotic Galileo spacecraft that soundlessly coasted past the Solar System's most photographed orb almost two decades ago. The Galileo spacecraft, although originally launched from Earth, coasted past its home world twice in an effort to gain speed and shorten the duration of its trip to Jupiter. During Galileo's first Earth flyby in late 1990, it made a majestically silent home movie of our big blue marble rotating by taking images almost every minute during a 25-hour period. The above picture is one frame from this movie -- clicking on this frame will put it in motion (in many browsers). Visible on Earth are vast blue oceans, swirling white clouds, large golden continents, and even one continent frozen into a white sheet of water-ice. As Galileo passed, it saw a globe that not only rotated but began to recede into the distance. Galileo went on to a historic mission uncovering many secrets and mysteries of Jupiter over the next 14 years, before performing a final spectacular dive into the Jovian atmosphere.

From 31 million miles away, how could you tell that there was life on Earth? Recently, scientists used the remote vantage point of NASA's Deep Impact spacecraft to shoot a sequence of images that will help to help answer that question—and will guide them in the search for planets outside our solar system that could also harbor life.

Shot at 15 minute intervals on May 28 and 29, 2008, the images have been stitched together into a movie that encompasses one full rotation of Earth. The video capture the transit of the Moon across an Earth backdrop that starts with the western Pacific in view. A few of the images in the sequence are shown above. The first image shows eastern Africa and the Arabian Peninsula. In the final image, the Atlantic Ocean has rotated into view. Other spacecraft have imaged Earth and the Moon from space, but Deep Impact is the first to show a transit of Earth with enough detail to see large craters on the moon and oceans and continents on Earth.

A far cry from the very detailed images of our planet that satellites closer to Earth provide, the video suggests what attributes of a life-supporting planet we could recognize from tens of millions of miles away. "Our video shows some specific features that are important for observations of Earth-like planets orbiting other stars," said Drake Deming of NASA's Goddard Space Flight Center in Greenbelt, Maryland. Deming is leading the search for extra-solar (outside our solar system) planets during Deep Impact's extended mission: a cruise to the Hartley 2 comet and a fly-by of it in November 2010.